Where transparent wall or ceiling windows are employed, such as in buildings and vehicles, it is often desirable to adjust from time-to-time the amount of incident light transmitted through the window, either for privacy or to regulate interior brightness. Opaque or translucent shutters, shades screens, drapes and blinds are popular conventional ways of adjusting transmittance. However, such shutters have a number of problems and disadvantages. For example, shutter structures located external to the window pane collect dust and need periodic cleaning, have exposed gears, pulleys, and cords that occasionally jam and need repairs, and occupy considerable additional space compared to the window pane itself. Also, three dimensional opaque structures, even when only partially closed, may unduly interfere with the user's view through the window.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention however, the following U.S. patents were considered related:
______________________________________ U.S. PAT. NO. INVENTOR ISSUED ______________________________________ 4,773,733 Murphy et al 27 Sept 1988 4,540,241 Rivier 10 Sept 1985 4,509,825 Otto et al 9 Apr 1985 4,285,577 Schuler 25 Aug 1981 4,123,141 Schuler 31 Oct 1978 3,663,089 Makas 16 May 1972 3,528,722 Makas 15 Sept 1970 3,504,962 Shanley 7 Apr 1970 ______________________________________
U.S. Pat. No. 4,773,733 (Murphy et al.) discloses a venetian blind having rotatable prismatic reflective slats so that direct rays of sunlight are excluded and indirect rays are admitted. However, this does not avoid the many general inconveniences of venetian blind structures. Another venetian blind structure, U.S. Pat. No. 4,509,825 (Otto et al.), in which the slats carry fresnel lens light reflective elements also has similar disadvantages.
A relatively compact light transmitting panel is disclosed in U.S. Pat. No. 4,540,241 (Rivier). This panel is a fixed assembly of transparent plates having inclined facets shaped and arranged so that incident solar radiation is refracted back toward the source or transmitted through the panel according to the angle of incidence. However, such a panel responds not to arbitrary adjustments made by the user, but only in a predetermined way to the angle of incident light.
A decorative type of compact window blind is shown in U.S. Pat. No. 3,504,962 (Shanley) in which a frame encloses two identical stationary light transmitting panels between which is a third, movable light transmitting panel. Each panel is arranged into a grid of alternating opaque and transparent rectangles. By moving the third panel between the first two, the user can achieve particular aesthetic effects, such as shading and coloring of the transmitted light or total opaqueness of the window. However, because of the checkerboard pattern of opaque and transparent rectangles, such a window cannot be made uniformly transparent.
It has been proposed in U.S. Pat. No. 3,528,722 (Makas) and 3,663,089 (Makas) to provide window structures that have two overlapping linear polarizer panes, one of which the user can rotate an arbitrary angle with respect to the other so that the light energy transmitted I obeys Malus' law EQU I=I.sub.max cos.sup.2 .theta. Eq. 1
where I.sub.max is the maximum amount of light transmitted and I is the amount transmitted when the direction of one of the linear polarizers is at an angle .theta. with respect to the direction of polarization of the other. That is, when the directions of polarization of the two linear polarizers are aligned (.theta.=0.degree.) the light transmitted I=I.sub.max is that which would be transmitted through either one of them alone. When the directions of polarization of the two linear polarizers are orthogonal to each other (.theta.=90.degree.) no light (I=0) is transmitted.
Assuming the incident light is unpolarized, the maximum transmitted light I.sub.max will be 50 percent of the incident light since half of the incident unpolarized light can be considered as not transmitted because polarized orthogonal to the direction of linear polarization of the first-encountered polarizer. There are many uses of windows where this theoretical maximum of 50 percent transmission of the incident light can be tolerated to gain the advantages of a polarizer-based transmittance-adjusting mechanism, in which for unpolarized incident light the transmittance is adjustable between 0 and 50 percent.
For convenience, hereafter when we refer to two overlapping linear polarizers that are aligned so that the light energy transmitted I=I.sub.max' we will say that the light transmitter they form is OPEN, even though its transmittance for unpolarized light is 50 percent. Note, that when two such overlapping polarizers are orthogonal to each other, the transmittance is 0, which means the light transmitter they form is OPAQUE.
Unfortunately, rotatable window structures as proposed in the Makas patents are useful in limited applications where windows of generally circular shape can be employed, such as in airplanes or ceiling windows. However, most windows are rectangular, in which case considerable extra space around the window would be required to accommodate rotation of at least one of the polarizer panes. In addition, in larger windows, such as in the sunroof of an automobile or in building windows, the power requirements for a rotatable window mechanism would be undesirably expensive.
Window structures that use polarizer panes without rotating them are shown in Schuler's U.S. Pat. Nos. 4,123,141 and 4,285,577. In these patents the window is divided into left and right halves and there is a movable half-window pane that carries at least a wave plate. The wave plate has a thickness selected to optically rotate the direction of polarization of incident light by 90.degree.. When the movable half-window pane is in a first half of the window, due to crossed polarization in each half of the window, no light is transmitted and the window is OPAQUE. When the user moves the half-window pane to the second half of the window, due to polarization alignment in each half of the window, light is transmitted and the window is OPEN. In effect, moving the wave plate form one half of the window to the other causes virtual relative rotation of the polarizers.
However, such a structure has the disadvantage of depending on the wave plate which constitutes an additional element beyond the polarizers. It is also a disadvantage that the waveplate must travel a considerable distance (half the window width), since this requires a relatively large actuating mechanism. In addition, the thickness of the wave plate must be an odd one-half-wave multiple of the wavelength of the incident light. Therefore, it must be fabricated with considerable precision and will respond differently to light of different wavelengths.
In view of the problems associated with the prior art, there is a need for a window structure that enables a user to easily adjust the transmittance of a window from OPAQUE to OPEN, yet, does not need a wave plate to rotate the direction of polarization of transmitted light. Further, there is a need for such a window structure that only requires a short travel distance for its components.